Re: Certificate proposal

[Hal <hfinney@shell.portal.com>]

tcmay@got.net (Timothy C. May) writes:
>For communication, the only credential Alice needs to ensure that only Bob
>can read her message is that she uses Bob's public key. If "Bob the Key"
>reads it, presumably it was "Bob the Person" who read it.

>(Again, Bob the Key = Bob the Person to many of us. If Bob the Person has
>let his private key out, so that Chuck the Person is also able to read the
>Bob the Key stuff, etc., then of course cryptography cannot really handle
>this situtation.)

OK, but again, what about the man in the middle attack?  Suppose the
key that you found that claims to be from Bob is actually not his, but
another one created by a man in the middle, such as Bob's malicious
ISP?  Then that ISP is decrypting the messages Alice sends to him using
that fake key, and re-encrypting them using Bob's real key.  He is
reading all of the messages, and Alice and Bob do not in fact have
communications privacy.

I don't want to overstate the risk of this attack.  It would not be an
easy one to mount and I believe there are countermeasures which could
detect it unless the MITM had nearly supernatural powers.  But the MITM
attack is normally considered seriously in discussing crypto protocols.
It is a well known weakness in Diffie-Hellman, for example.  That is why
authenticated Diffie Hellman is used in some of the newly proposed key
exchange protocols for IP.  The risks of MITM attacks on public key
systems was recognized not long after those systems were proposed.  The
problems with fake keys have been discussed for over a decade.

Why is this all suddenly irrelevant?  Were these attacks never realistic?
Is it just not a problem somehow?  I am baffled by the fact that people
are just turning their backs on all these years of research and
experience.  If this is some kind of paradigm shift in which the idea of
communicating with keys is seen as the key to the puzzle, then I am
afraid I don't share the enlightenment.  To me the problem seems as real
as ever.

Hal